An Iranian surface-to-air missile destroyed a US Global Hawk surveillance drone on Thursday. Tehran said the drone was shot down over its territory and Washington said it occurred in international airspace over the Strait of Hormuz.

The incident aggravated fears of a direct military clash between the longtime foes. Oil futures rose more than 1% to above $US65 ($A94) per barrel on Friday over worries about possible disruptions to crude exports from the Gulf.

Trump's decision to cancel what he said was a planned attack on three sites suggests he wants a diplomatic solution to end weeks of festering tensions with Iran, which Washington accuses of a spate of attacks on oil tankers in the Gulf region.

"I'm not looking for war, and if there is, it'll be obliteration like you've never seen before. But I'm not looking to do that," Trump told NBC News in an interview aired on Friday night.

Iranian sources told Reuters that Trump had warned Tehran via Oman that a US attack was imminent, but had said he was against war and wanted talks. Washington also requested a closed-door UN Security Council meeting on Monday.

The State Department denied the Reuters report. "Reports that a message was passed last night to the Iranians via an Omani back channel are completely false. These reports are pure Iranian propoganda," department spokeswoman Morgan Ortagus said on Twitter.

In a series of early-morning tweets, Trump said he was in no hurry to launch a strike and that US economic sanctions designed to force Iran to curb its nuclear and missile programs and its involvement in regional wars were having an effect.

"We were cocked & loaded to retaliate last night," Trump tweeted.

"Ten minutes before the strike I stopped it, not proportionate to shooting down an unmanned drone. I am in no hurry, our military is rebuilt, new, and ready to go, by far the best in the world," Trump tweeted.

Typically, the US military would seek to target Iranian facilities that could be connected to the shooting down of the American drone. The military would not seek to inflict casualties and although Iranian military casualties could occur in an attack, forecasts such as 150 are normally only rough estimates.

White House national security adviser John Bolton, Secretary of State Mike Pompeo and CIA Director Gina Haspel, along with the rest of Trump's team, favoured a retaliatory strike, a senior Trump administration official said.

"There was complete unanimity amongst the president's advisers and DOD (Department of Defence) leadership on an appropriate response to Iran's activities. The president made the final decision," said the official.

Trump's decision drew mixed reviews in Washington, with some people criticising him for flinching while others, notably senior Democrats, praised what they saw as restraint.

"A strike of that amount of collateral damage would be very provocative, and I'm glad the president did not take that," House Speaker Nancy Pelosi, the top Democrat in Congress, told reporters.

However, Michael Makovsky, a former Pentagon official who heads the Jewish Institute for National Security of America, a think tank that favours strong US-Israeli security ties, said Trump was undermining US credibility.

"Trump has given the impression he lost his nerve," Makovsky said in a statement.

Iran's destruction of the US drone was the latest among ever more serious incidents in the Gulf region, a critical artery for global oil supplies, since mid-May.

There have also been explosions on six oil tankers that the US suspects Iran or its proxies of carrying out. Iran has denied any involvement.

Australian Associated Press